I-III-VI compound semiconductor materials have received significant attention for use in photovoltaic cells because of the desirable band gap provided by many of those materials. Several photovoltaic cell designs have been proposed using such semiconductors, with a common cell design incorporating a p-type layer of the I-III-VI semiconductor material forming a heterojunction with an n-type semiconductor material, such as cadmium sulfide or cadmium zinc sulfide.
A number of different I-III-VI semiconductor materials have been proposed for use in photovoltaic cells. Some examples include AgInS.sub.2, AgGaSe.sub.2, AgGaTe.sub.2, AgInSe.sub.2, AgInTe.sub.2, CuGaS.sub.2, CuInS.sub.2, CuInTe.sub.2, CuAlS.sub.2, and CuGaSe.sub.2. Most attention, however, has been focused on copper indium diselenide (CuInSe.sub.2) and variations of copper indium diselenide in which a portion of the indium is replaced with one or more of aluminum and gallium and/or a portion of the selenium is replaced with sulfur and/or tellurium. Copper indium diselenide is commonly referred to as "CIS." Two promising variations of CIS that have been proposed include CuIn.sub.x Ga.sub.1-x Se.sub.2 (commonly referred to as "CIGS") and CuIn.sub.x Ga.sub.1-x Se.sub.y S.sub.2-y (commonly referred to as "CIGSS"). These and other I-III-VI semiconductors may be manufactured for use in photovoltaic cells according to the present invention.
A variety of techniques have been proposed for fabricating various I-III-VI semiconductor materials for use in photovoltaic cells. One technique is to coevaporate all of the I, III and VI components and then react the components to form the desired semiconductor material. This technique has been used to produce small-area photovoltaic cells with very high efficiencies, in excess of 15%, but the technique is complex. Simultaneous deposition from a number of sources is difficult to control, and scale-up for a commercial operation is a significant problem.
Another technique is to sequentially deposit precursors for the desired semiconductor material and then heat the deposited precursors to form the desired semiconductor material. Relative to coevaporation, sequential deposition is less complex and easier to control and is, therefore, easier to scale up for a commercial operation. A significant problem with sequential deposition techniques, however, is that resulting photovoltaic cells tend to have relatively low efficiencies.
One variation on the sequential deposition technique for making CIS involves "selenization" of predeposited copper and indium films to form the final semiconductor material. This selenization is accomplished by reacting the metal precursors with a reactive selenium-containing gas, typically hydrogen selenide. Although the selenization process has resulted in somewhat improved photovoltaic cell efficiencies, hydrogen selenide is a very hazardous gas, which makes operation of the process problematic.
Another significant problem with all of the fabrication techniques, and especially with sequential deposition techniques, is a lack of good adhesion of the I-III-VI semiconductor material to adjoining layers, and especially to a molybdenum film often used as a back contact for CIS photovoltaic devices. Without good adhesion, delaminations can occur which significantly impair the performance and reliability of the photovoltaic cells.
A need exists for new techniques to fabricate I-III-VI semiconductors for use in photovoltaic cells, and especially for fabrication techniques that are easily scaled up for use in a commercial operation, that permit the manufacture of photovoltaic cells having reasonably high efficiencies, that avoid the use of highly hazardous gases and that promote good adhesion of the semiconductor material to adjoining layers.